If I start to build a watchtower on a flat ground with a small inclination, the side of the tower that faces the higher ground cannot be accessible and the building actions do not appear.

As if the interaction spot is buried underground.

Also, I cannot place materials directly when holding them in my hands, if I place the materials using the GUI, I still cannot get the "build base" action when holding a hammer.

There are no obstacles around or above the towers, nothing to appear to block it.

When building a fence exactly on the same spot - it works.

The specific area where this happened is at the red house that stands alone in an open field south-east to Gvozdno.
This building has a small broken fence around it. The spots are in the north-east and south-east corners of the ground that is around this house, outside the small fence.
Also in the area beween the house (lower exit to the south) and the line of trees there.